The long term goal of our research is to understand the biochemical and molecular process underlying immune mediated demyelination occurring in diseases such as multiple sclerosis and Guillain Barre syndrome. We have previously focused our studies on pathobiology of demyelination induced by complement, especially the membrane-reactive terminal complement complexes (TCC). Experimental results during the preceding grant period are summarised; We found that myelin activates complement by direct C1 binding, and allows TCC to form, which is required for demyelination of myelinating cultures. The TCC may cause splitting myelin lamellae and myelin vesiculation, since effective hydrolysis of structural myelin proteins such as MBP can be induced by TCC through activation of myelin proteases. In oligodendrocytes (OLG), TCC mobilizes arachidonic acid (AA) and LTB4. In addition, sublytic TCC was found to selectively reduce the MRNA accumulation encoding proteolipid and MBP (but not beta-actin). This TCC effect was also seen in the presence of transcription inhibitor, indicating accelerated MRNA decay. Thus, myelin formation, an important function of OLG, can be affected by complement. In this application, we will study the mechanisms of AA mobilization by exploring signal messengers required to activate lipases responsible for AA production by TCC. The efficiency of myelin phagocytosis by macrophages mediated by complement-derived opsonic peptides, C3b and iC3b, will be evaluated. We found that myelin is devoid of DAF, membrane protein which down-regulates complement cascade. Therefore, increased opsonization of myelin by C3b and iC3b will enhance macrophage-mediated damage and clearance of myelin through interaction with complement receptors, CR1 and CR3, expressed on macrophages. In addition, molecular mechanisms involving post-transcriptional regulation of myelin protein genes by TCC will be investigated. The RNA Sequence-specific motif(s) which is responsible for the accelerated MRNA decay, in responds to the signal induced by TCC, will be identified. Finally, the role of the complement system in immune-mediated demyelination will be examined in vivo. Specifically, the role of C1 activation directly by myelin in vivo following breakdown of blood brain barrier will be examined as well as the role of TCC in inflammatory demyelination.